4.2 Codominance, Multiple Alleles and Incomplete Dominance

In codominance, both allele products are detectably expressed in a heterozygote. In incomplete dominance, the heterozygote has an intermediate phenotype. Neither changes segregation: a heterozygote still puts one allele in each gamete type. The distinction concerns how genotype becomes phenotype.

The ABO blood-group locus has three common population alleles: $I^A$, $I^B$ and $i$. A diploid individual carries only two. $I^A$ and $I^B$ are codominant with each other and each dominates $i$. Thus $I^AI^A$ and $I^Ai$ give A, $I^BI^B$ and $I^Bi$ give B, $I^AI^B$ gives AB, and $ii$ gives O.

A gene having multiple alleles means more than two variants exist across the population, not that one person carries all of them. This expands possible genotypes without altering diploidy. ABO phenotype A or B does not identify a unique genotype, whereas AB and O do in this simplified model.

In the Marvel of Peru, red and white homozygotes produce pink heterozygotes: an intermediate output. By contrast, AB red blood cells display both A and B antigens. Codominance preserves two distinguishable products; incomplete dominance gives an intermediate measured phenotype.

A genotype–phenotype rule assumes a biological environment. Temperature affects enzyme activity, nutrition limits growth, and regulatory variants alter expression. Letter notation models inheritance, not isolation from all other genes and conditions.

State the allele relationship before assigning phenotypes. Use superscripts for codominant alleles so neither is written as recessive. Do not call every mixed phenotype codominance: ask whether both parental products remain distinguishable or whether the measured result lies between them.

ABO inheritance is best solved by working backward from informative offspring. A group O child must be ii and must receive i from each parent. A group AB child must receive $I^A$ from one parent and $I^B$ from the other. A group A parent who produces an O child cannot be $I^AI^A$; the parent must be $I^Ai$. These deductions can identify parental genotype without testing every possible cross.

A cross between $I^Ai$ and $I^Bi$ produces four equally likely genotypes, but a cross between $I^AI^B$ and $ii$ produces only $I^Ai$ and $I^Bi$. No child in the second cross can be AB because the group O parent supplies only i, and none can be O because the AB parent supplies no i. The phenotype of each parent is not enough; the gamete set is the decisive link.

Incomplete dominance often changes an F2 phenotype ratio from 3:1 to 1:2:1 because every genotype has a distinguishable phenotype. The genotype ratio did not change. This is a useful diagnostic: dominance can merge AA and Aa into one phenotype class, but it cannot alter the probability that those genotypes form. Whenever phenotype and genotype ratios differ, identify which genotypes have been grouped together.